Fuel cell system for low pressure operation

ABSTRACT

This invention is an improved fuel cell design for use at low pressure. The invention has a reduced number of component parts to reduce fabrication costs, as well as a simpler design that permits the size of the system to be reduced at the same time as performance is being improved. In the present design, an adjacent anode and cathode pair are fabricated using a common conductive element, with that conductive element serving to conduct the current from one cell to the adjacent one. This produces a small and simple system suitable for operating with gas fuels or alternatively directly with liquid fuels, such as methanol, dimethoxymethane, or trimethoxymethane. The use of these liquid fuels permits the storage of more energy in less volume while at the same time eliminating the need for handling compressed gases which further simplifies the fuel cell system. The electrical power output of the design of this invention can be further increased by adding a passage for cooling the stack through contact with a coolant.

This application is a divisional application of co-pending U.S. patentapplication Ser. No. 09/523,910, filed on Mar. 13, 2000, which was adivisional of U.S. application Ser. No. 08/926,547, filed Sep. 10, 1997,now U.S. Pat. No. 6,054,228, which was a continuation-in-part of U.S.application Ser. No. 08/656,968, filed Jun. 6, 1996, now U.S. Pat. No.5,709,961.

FIELD OF THE INVENTION

This invention relates generally to the field of fuel cells. Moreparticularly, the invention relates to low pressure fuel cells and theircomponents.

BACKGROUND OF THE INVENTION

Generally, a fuel cell is a device which converts the energy of achemical reaction into electricity. Fuel cells differ from batteries inthat fuel and oxidant are stored external to the cell, which cangenerate power as long as the fuel and oxidant are supplied. A fuel cellproduces an electromotive force by bringing the fuel and oxidant incontact with two suitable electrodes separated by an electrolyte. Afuel, such as hydrogen gas, is introduced at one electrode where itdissociates on the electrocatalytic surface of the positive electrode(anode) to form protons and electrons, as elucidated in equation 1. Theelectrons pass into the conductive structure of the electrode, and therefrom to the external electrical circuit energized by said fuel cell. Theprotons formed by dissociation of the hydrogen at the first electrodeand pass through the electrolyte to the second electrode.Simultaneously, an oxidant, such as oxygen gas or air, is introduced tothe second electrode where it is adsorbed on the electrocatalyticsurface of the negative electrode (cathode) and is electrochemicallyreduced to form a surface oxide species by electrons having transversedthe external electrical circuit energized by the fuel cell. This surfaceoxide reacts with protons from the electrolyte to form water, theproduct of the net reaction. The water desorbs from the electrode andleaves the cell in the cathode gas stream. The half cell reactions for ahydrogen consuming fuel cell at the two electrodes are, respectively, asfollows:H₂→2H⁺+2e⁻  (1)½O₂+2H⁺+2e⁻→H₂O   (2)

Connecting the two electrodes through an external circuit causes anelectrical current to flow in the circuit and withdraws electrical powerfrom the cell. The overall fuel cell reaction, which is the sum of theseparate half cell reactions written above, produces electrical energyand heat.

In practice, fuel cells are not operated as single units, but areconnected in a series to additively combine the individual cellpotentials and achieve a greater, and more useful, potential. The cellsin a given series can be connected directly, with opposing faces of asingle component in contact with the anode of one cell and the cathodeof an adjacent cell, or through an external electrical linkage. A seriesof fuel cells, referred to as a fuel cell stack, are normally equippedwith a manifold system for the distribution of two gases. The fuel andoxidant are directed with manifolds to the correct electrodes, andcooling is provided either by the reactants or by a cooling medium. Alsowithin the stack are current collectors, cell-to-cell seals, and othercomponents. The stack and associated hardware make up the fuel cellmodule.

In fuel cells where a solid polymer electrolyte or proton exchangemembrane (APEM≅) is used, the membrane acts as the electrolyte as wellas a barrier for preventing the mixing of the reactant gases. A PEM fuelcell is described in greater detail in Dhar, U.S. Pat. No. 5,242,764,which is incorporated herein by reference. Much research and developmenthas been devoted to improving the power-to-weight ratio for protonexchange membrane (APEM≅) fuel cells. Most of this research has involvedincreasing the power per unit volume of relatively heavy stacks.

FIG. 1 is a drawing illustrating a fuel cell stack based on aconventional bipolar filter press design 10 with graphite structureelements. A full description of filter press type fuel cells may befound in Neidrach, U.S. Pat. No. 3,134,697 which is incorporated hereinby reference. While improvements in the filter press style fuel cellshave provided significant increases in power per unit volume, theoverall systems that have evolved are large, heavy, and relativelycomplex, with compressors to supply air and pumps to provide forcedwater cooling systems to remove excess heat.

FIG. 2 shows the structure of a standard fuel cell membrane andelectrode (M&E) assembly 20 intended for use in the bipolar stack 10 ofFIG. 1, which has current collection over most of the back of theelectrode. The M&E assembly consists of a membrane 22, a catalyst layer24, a gas diffusion layer 26 and a conductive cloth backing 28. Asillustrated, a complete M&E assembly includes similar layers formed onboth sides of the membrane.

More recently, efforts have been made to reduce the stack weight byreplacing the heavy carbon elements with thinner and lighter, metalelements. However, these units were designed for large scaleapplications, some on the order of about 30 kW, and, therefore, requirethe same stack ancillary equipment mentioned above. Furthermore, theancillary equipment included with the stack in these systems has beendesigned to operate efficiently at the kilowatt level. Scaled downversions of these systems have been attempted in applications thatrequire much less power, such as within the range between about 50 andabout 150 Watts. However, these systems are not well suited for stackoutputs in the tens or hundreds of watts, since the rotating components,such as pumps and compressors, do not scale down well. As a result, evensmall scale systems of this design are too heavy for many smallapplications, such as for portable applications and personal use.

Therefore, perhaps the most important objective for portable andpersonal applications is not Watts per unit volume but Watts per unitweight, i.e. W/lb. Efforts to adapt the standard bipolar filter pressdesign to low pressure operation, thereby eliminating much of theancillary equipment, have met with some limited success, producingstacks with power densities as high as 61 W/lb. While this is a usefulpower density, these systems require complicated and expensive assembly.

One possible way of improving fuel cell systems for operation at lowerpressures is using liquid fuels in lieu of gaseous fuels, such ashydrogen. Methanol (CH₃OH) and other related compounds, such asdimethoxymethane (C₃H₈O₂) and trimethoxymethane (C₄H₁₀O₃), offer verypromising alternatives to gaseous fuels.

The liquid fuels mentioned above share some common advantages comparedto hydrogen. First, they are all pourable liquids at ambient pressureand ambient, and near ambient, temperatures. Second, they have a muchhigher energy density than hydrogen. For example, a 1:1 methanol:watermixture (each mole of methanol requires a mole of water forelectrochemical oxidation, as shown in Equation 3) has as much potentialenergy as hydrogen stored at a pressure of 16,000 psi.CH₃OH+H₂O+{fraction (3/2)}O₂+3 H₂O tm (3)

Although there is a net production of water, e.g., more water isproduced at the cathode than consumed at the anode, water must besupplied to the anode because in a fuel cell the oxidation is carriedout as a pair of half cell reactions, with Equation 3 representing thenet reaction. In a fuel cell water is consumed in the anode reaction(Equation 4) and produced in the cathode reaction (Equation 5).CH₃OH+H₂O→CO₂+6H⁺+6e⁻  (4)6H⁺+6e⁻+{fraction (3/2)}O₂→3 H₂O   (5)

The reactions for dimethoxymethane and trimethoxymethane are similar,with four and five molecules of water needed for each molecule of theoxidized organic compound respectively. Since they are consumed athigher water-to-fuel stoichiometries, the two ethers should be presentat lower concentrations in the fuel stream, and consequently have lowerpermeation rates through PEM membranes and lower vapor pressures overthe solution. This is especially true in the case of trimethoxymethane,which has a boiling point of 104° C., 40° C. higher than methanol, andconsequently, a lower vapor pressure than methanol under all conditions.The lower vapor pressure, combined with lower toxicity for thesecompounds, leads to less vapor toxicity hazards compared to thoseassociated with the use of methanol.

Therefore, there is a need for a lightweight fuel cell system thatprovides an improved power density (W/lb) and eliminates much of theancillary equipment. There is also a need for fuel cells that operate ongaseous fuels, such as hydrogen, and fuel cells that operate on liquidfuels, such as dimethoxymethane. It would be desirable if the fuel celloperated efficiently in the 50 to 150 Watt range to supply electricityto a variety of common electrical devices. It would also be desirable ifthe fuel cell had no more than a few moving parts to reduce maintenanceand avoid breakdowns. It would be further desirable to have a fuel cellsystem that was available in modules that could be configured togetherto meet the power requirements of specific applications.

SUMMARY OF THE INVENTION

The present invention provides an apparatus comprising an array ofcells. Each cell comprises a membrane, an anode comprising a conductiveelement, and a cathode comprising a conductive element. The cathode ofone cell shares its conductive element with the anode of an adjacentcell; the sharing of conductive elements establishing an electricalconnection between the cells in the array. In one embodiment the cellsare electrochemical cells suitable for the direct production ofelectricity through the simultaneous oxidation of a fuel, or reducingagent, at one electrode, the anode, accompanied by simultaneousreduction of an oxidizer at the other electrode, the cathode. In anotherembodiment, the cells can be used to produce oxidized and/or reducedproducts through simultaneous electrochemical oxidation at one electrodeand reduction at the opposing electrode of a compound or a mixture ofcompounds by application of an electrical current Further, the inventionprovides for the use of both gas and liquid feeds.

The invention also encompasses a low pressure gas electrode comprising(a) an electrically conducting supporting substrate; wherein saidsubstrate comprises at least two opposite faces; a plurality of openingsallowing passage from one face to an opposite face; said openingsencompassing between about 10% and about 90% of the total area of thesubstrate; and (b) an electrically conductive porous material bound tothe substrate comprised of a conductive component and a bindingcomponent. In one embodiment, the conductive porous material is amixture comprising a powdered component and a fibrous component. In yetanother embodiment, the powdered and/or fibrous components comprisecarbon.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the above recited features and advantages the present inventioncan be understood in detail, a more particular description of theinvention, briefly summarized above, may be had by reference to theembodiments thereof which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a standard filter press type fuel cell stack 10 showing thearrangement of the bipolar cell plates and end plates.

FIG. 2 is a cross-sectional view of a standard PEM fuel cell membraneand electrode assembly (M&E) 20.

FIG. 3 illustrates the key components for a monopolar cell stack,including a terminal cell frame 36, a flat bipolar cell frame 30 and acurved bipolar cell frame 38.

FIG. 4 is a cross-sectional view of a fuel cell stack 50 comprised ofmonopolar cells based on the use of the bipolar cell frame components ofFIG. 3.

FIG. 5 is a graph showing the effect of both increased fuel pressure andforced convection on the performance of a four cell fuel cell stacks

FIGS. 6(a-c) are cross-sectional views of three configurations for anM&E produced with a metal conductive element (grid) 82 included as partof the structure.

FIG. 7 is a graph showing the improvement in performance produced byattaching a metal grid to the back of the electrode.

FIG. 8 is an exemplary monopolar fuel cell stack 95.

FIG. 9 is a perspective view of a compact fuel cell 100 having a fan 106to provide forced convention of air over the cathodes 108.

FIG. 10 is an illustration of the components of a monopolar fuel cellconstructed without cell frames.

FIG. 11 is an expanded metal sheet suitable for use as the conductiveelement in the electrodes.

FIG. 12(a) is a face on view into the cathodes on one face of a fin.

FIG. 12(b) is a cross-sectional view cut vertically near the center ofFIG. 12(a).

FIG. 12(c) is a view of an enlarged piece cut out of the side of 12(b)indicating the location of the wicking structure.

FIG. 13(a) is a drawing showing a wrap-around fuel cell stack supportedon its fuel supply, illustrated here as a cylinder with series of squarewindow panes on the surface of the cylinder.

FIG. 13(b) is a drawing showing a wrap-around fuel cell stack supportedon its fuel supply, illustrated here as a cylinder with a series ofbands, with long, narrow individual cells.

FIG. 14 is a cross-sectional view of a monopolar fuel cell mounted onthe outside of the vessel that supplies its fuel.

FIG. 15 is a graph showing polarization curves for M&E's produced usingthree different conductive elements in the electrodes with the samecomposition for the gas diffusion layer and the catalyst layer.

FIG. 16 is a graph showing polarization curves for M&E's produced usingtwo different membrane materials with the same conductive element (Cu{fraction (4/3)}) and the same compositions for both the gas diffusionlayer and the catalyst layer.

FIG. 17 is an illustration of the arrangement used to focus the pressload on one cell of a multi-cell module.

FIG. 18 is a cross-sectional view of an electrode fabricated using gasdiffusion electrodes with a metal grid as part of the electrode forimproved internal conductivity.

FIG. 19 is graph showing the polarization curve for a two-cell monopolarmodule with the cells connected through their common electrode support

FIG. 20 is a graph showing polarization curves for a three-cellmonopolar fuel cell module illustrating the effect of fuelhumidification on the unit's performance.

FIG. 21 is a graph showing the power output of the three-cell monopolarfuel cell module shown operating without humidification in FIG. 20.

FIG. 22 is a graph showing polarization curves for a three-cellmonopolar fuel cell module illustrating the effect of hydrogen pressureon the unit's performance.

FIG. 23 is a graph showing polarization curves for a three-cellmonopolar fuel cell module illustrating the effect of air motion on theunit's performance.

FIG. 24 is a graph showing the polarization curves for a stack operatedwith and without air flow from a fan.

FIG. 25 is a schematic diagram depicting a polymeric frame.

FIG. 26(a) is a cross sectional view of a segment of a fin similar tothe fin of FIG. 31(b) with an internal heat transfer structure added.

FIG. 26(b) is an edge view of the fin of FIG. 26(a) showing the internalheat structure.

FIG. 27 is a drawing depicting a wavy convoluted aluminum sheet as anexample of internal heat transfer structures.

FIG. 28 is a cross sectional view of the fin of FIG. 26(a) with the backplane of the monopolar cells extended and folded over to form aprotective cover.

FIG. 29 is a cross sectional view of a fin with protective cooling fins.

FIGS. 30(a-c) are cross-sectional views of three configurations for anM&E produced with a metal conductive element included as part of thestructure.

FIG. 31(a) is a face on view into the cathodes on one face of a fin.

FIG. 31(b) is a cross-sectional view cut vertically near the center ofFIG. 21(a).

FIG. 31(c) is a cross-sectional view cut horizontally near the center ofFIG. 31(a).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides an apparatus that is useful inelectrochemical cells, especially those useful for generatingelectricity known as fuel cells. More particularly, the inventionprovides various electrode structures or configurations that provideincreased power output per unit weight of the cell. The electrodes maybe used with gaseous fuels, such as hydrogen, or liquid fuels, such asmethanol. The electrodes are designed to operate even at low fuel andoxidant pressures.

One aspect of the invention provides an electrically conductive memberdisposed within the electrode, rather than having a conductive clothlayer and a conductive frame disposed along a perimeter face of theelectrode. The electrically conductive member may be a sheet of expandedmetal mesh or wire, preferably expanded metal mesh. The expanded metalmesh or other electrically conductive member preferably has a greaterportion of open area than does carbon cloth or paper to increase the gasflow to and from the catalyst areas.

Another aspect of the invention provides a wicking member for deliveringa liquid fuel to the anode side of the fuel cell or to theelectrochemical cell. The liquid fuel may be mixed with variousconcentrations of water as necessary to the facilitate the electrodereactions and provide water to the proton exchange membranes.

A further aspect of the invention provides a cooling member disposedbetween adjacent cells. While cooling may be accomplished by externalair circulation, either natural or forced, this is sometimes inadequate,and it is sometimes advantageous to add a cooling member. The preferredcooling members of the invention comprise air channels disposed betweenfuel feed ports in the anode compartment. The channels may be as simpleas a pair of walls with an opening therebetween or the channel maycontain one or more heat transfer members.

The present invention provides a lightweight fuel cell system thatoperates with fuel and oxidant gases at near atmospheric pressures. Thefuel cell system uses a monopolar cell design where the electrodesurfaces are sufficiently accessible to the gases that it is notnecessary for the oxidizer and reducer gases to be compressed. The fuelcell is ideal for personal use due to its light weight, compact size andself contained operation An added feature of the present invention isthat individual fuel cells may be linked together to achieve greatervoltages or currents.

The present invention provides a monopolar fuel cell system based on theuse of flat cell frames that support the cell components. A monopolardesign is lighter than a bipolar or filter press design for a number ofreasons. First, since the monopolar fuel cell is not under compression,the components, such as the cell frames, can be much thinner than thestructure of a bipolar fuel cell which relies on the thickness of thecomponents to maintain precise spacings for gas flow therethrough. Theprimary limitation on the thickness of the components is that thecomponent in question must have sufficient strength to maintain itsintegrity during use in various applications. This permits the use of awide variety of lightweight conductive materials, such as a thintitanium sheet.

Another reason that the monopolar fuel cell of the present inventionweighs less than conventional systems, is that many components of aconventional system are no longer necessary and may be eliminatedentirely. For example, the preferred monopolar fuel cell uses ambientair as the oxidizer, rather than compressed air or oxygen, as theoxidizer. Because it is unnecessary to compress the oxidizer, theconventional gas control system is unnecessary and may be eliminated.Instead, the gas is supplied to the cathode by either free or forcedconvection. In either case, there is no need for an enclosed cathodeflow field.

In addition, the bipolar plates of conventional fuel cells, whichseparate anode and cathode flow fields and conduct current from one cellto the next, are no longer needed and may be eliminated. The plates arenot needed to separate flow fields because the anode and cathode flowfields of the present invention are open to the sealed reducing gaschamber and the atmosphere, respectively, and are effectively separatedby the cells themselves. Furthermore, the bipolar plates are not neededto conduct electrical current from one cell to the next since thisfunction is performed by a different electrically conducting member,such as an external bipolar frame or an internal bipolar grid

Another component or system of conventional fuel cell Systems that is nolonger required is the heat transfer system. The fuel cell system ofthis invention has a much larger ratio of exposed area to active areaand, therefore, dissipates waste heat directly to the surrounding airwithout the need for a circulating heat transfer fluid, a pump tocirculate the fluid, or heat exchangers inside the stack to collect theheat and outside the stack to dissipate the heat. However, in someapplications, it may still be desirable to include a cooling member suchas an air channel made from a material having a high thermalconductivity.

Eliminating moving parts from fuel gas systems, in accordance with thepresent invention, eliminates the parasitic power losses required todrive the motors involved and the potential for malfunctions.Furthermore, eliminating small motors and moving parts in fuel gassystems reduces maintenance and malfunctions, particularly inapplications where the system is portable.

Another advantage of monopolar fuel cell systems is the low cost ofmanufacture. Because the entire stack is assembled from a series offlat, or nearly flat, components, all of the parts can be cut from flatsheets. No complex machining steps are required.

Electrochemical Cells Having External Bipolar Frames

The membrane and electrode assemblies used in the present invention arecomprised of a proton exchange membrane (APEM≅), catalyst layers oneither side of the PEM, and gas diffusion layers over the catalystlayers. Gas diffusion electrodes, their construction and fabrication,are generally described in Murphy et al., U.S. Pat. No. 5,460,705 whichis hereby incorporated by reference. The preferred PEM is generally apolymer material having sulfonate functional groups contained on afluorinated carbon backbone, such as the perfluorinated sulfonic acidpolymers available under the trade name NAFION from Du Pont de Nemours,E. I. & Co of Wilmington, Del. The gas diffusion layers preferablycomprise a carbon conducting paper or cloth. In one preferredembodiment, a platinum catalyst is supported on the carbon paper beforehot pressing with the PEM. However, it is also anticipated that the samegeneral construction can be achieved using thin film electrodes, wherethe catalyst is deposited directly onto the PEM rather than the carbonpaper.

It is preferred that the reducing gas be delivered to the anodes from asupply vessel through a pressure regulating feed valve and feed lineconnected to the reducing gas chamber. The anode surface of eachindividual cell is directly exposed to the reducing gas in the chamberand does not require enclosed anode flow fields for delivery of thereducing gas. Since the reducing gas chamber allows the reducing gas toflow freely over the anode surface, it is not necessary to compress thereducing gas for delivery through narrow passages or flow fields.Therefore, the reducing gas may be supplied at any pressure. However, itshould be noted that while the fuel cell does not require the reducinggas, such as hydrogen, to be pressurized, it is generally preferred thatthe hydrogen is stored in a pressurized vessel that can be transportedalong with the fuel cell itself. It is further preferred that thepressurized hydrogen be delivered from the vessel to the reducing gaschamber through a step-down regulator to a pressure generally belowabout one atmosphere, but most preferably below about 2 psi to avoiddisplacing the membrane and electrode assemblies out of their frames.

The fuel cell consumes the fuel at the anode, produces water at thecathode and generates a flow of electricity for use in variousapplications. The water that is generated at the cathode is useful tokeep the PEM moist so that it will conduct protons efficiently. Waterwill evaporate from the cathode surface into the air and provide somecooling to the cells. However, because there is no external source ofwater to the PEM, the air flow rate and temperature should not beallowed to dry out the PEM.

In one aspect of the invention, it is possible to fabricate the cellswith the same catalyst, preferably platinum (Pt) or aplatinum-containing alloy, and the same catalyst loading on both theanode and the cathode. Therefore, the cells may be cut, handled andfabricated without regard for their orientation. This may beadvantageous in some applications, since the anode and cathode cannot bedistinguished by appearance alone. On the other hand, because Pt hasgreater catalytic activity for hydrogen gas than for oxygen gas, a cellwith better precious metal utilization is obtained when the platinumloading is lower on the anode than on the cathode.

The sheets and other components of the present invention may beassembled and coupled in many different ways and with many differentmeans as will be recognized in the art In particular, it should berecognized that the components may be fastened with mechanicalfasteners, such as bolts, screws, clamps, straps, rivets and the like,adhesives, and welding of the components. However, it is most preferredthat all of the components be fastened using adhesives and welding, sothat a minimal amount of material is added to the fuel cell and aminimal number of parts are needed. Bonding processes are also generallypreferred since this type of assembly can easily be automated.

Now referring to FIG. 3, a face view of three types of cell framecomponents are shown. First, there is the dual cell frame 30 which hasopenings 32, 34 for exposing two M&E assemblies. The dual cell frames 30are overlapped (as shown in FIG. 4) so that the opening 32 of a firstframe 30 coincides with the opening 34 of an adjacent frame 30. The twometal frames 30 hold the M&E assemblies firmly around the edge of theassembly.

Now referring to FIG. 4, a top cross-sectional view of a fuel cell stack50 is shown. The overlapping metal frames 30 can be repeated any numberof times to form the multi-cell stack or fin 51. At the end of theseries of cells, a terminal cell frame 36 may be used to overlap one ofthe openings 32, 34 and also to provide an electrical contact 37 forconducting electricity to and from the stack. The curved dual cell frame38 has two openings 40, 42 like the dual cell frame 30, except that ithas an enlarged central region 44 which can be curved to form the distalend of a fin 51. The fin 51 can have any number of cells 52, but isshown here having eight cells connected in series. In a fashion similarto the M&E assembly of FIG. 2, the cells 52 have an M&E assemblycomprised of a central PEM 54, a catalyst coating 56 and gas diffusionlayer 58.

Side plates 60 are sealed to the top and bottom edges of the fin 51 andan end cap 62 is sealed to the open end of the fin 51. The plates 60 andcap 62, in cooperation with the fin 51, form a chamber 64 incommunication with all of the anode surfaces 66. A gas inlet port 68 isprovided in the end cap 62 for the introduction of a fuel to the chamber64.

The two-sided arrangement shown in FIG. 4 represents a preferredembodiment of the invention. This arrangement permits the largest numberof cells to share a common internal fuel gas chamber. It is alsopossible to arrange the series of cells as a flat strip, where theopposing surface of the internal chamber is an inactive surface. Whilegenerally less efficient in terms of weight or volume, in someapplications, this may be a preferable arrangement.

The preferred fuel is a reducing gas, with the most preferred gas beinghydrogen In operation, each of the individual cells of the fuel cell 50has a cathode surface 70 that is exposed to the air as the oxidizinggas. Therefore, the fuel cell 50 operates at low pressures and withoutany moving parts. The cell is also lightweight, easy to manufacture andrelatively inexpensive.

Unlike a conventional bipolar system, this fuel cell collects thecurrent only from the edges of the electrode, leaving the entire faceopen. Leaving the area of the electrode as open as possible reduces thepotential for interference with diffusion and facilitates the supply ofair under ambient conditions without the need for a compressor or otherhigh powered air moving device.

A four cell version of fuel cell 50 of the present invention wasconstructed and operated using hydrogen gas as the fuel and ambient airas the oxidizer. The performance of the fuel cell is illustrated in FIG.5 by a graph of stack potential (Volts) as a function of stack current(Amps). As is apparent from the data displayed in that figure,increasing the fuel pressure slightly (1.5 psig) and using a small fanto create a forced convection flow over the stack produced a markedimprovement in the stacks performance.

Now referring to FIGS. 6(a-c), the structures of three different M&Eassemblies specifically intended for use in a monopolar fuel cell areshown. In the configuration of FIG. 6(a), a metal grid 82 is embedded inthe front surface of the electrode 80 and in direct contact with boththe electrocatalytically active portion 56 of the electrode and themembrane 54. The advantage of this arrangement is good electricalcontact with the electrocatalyst for efficient current collection andthe least interference with gas diffusion within the electrode. Thedisadvantages are the difficulty in making good electrical contactbetween the current collection plates 30 and both the metal grid 82 andthe conductive carbon cloth 84 at the back of the electrode, thepotential for damaging the membrane 54 with the grid while attaching theelectrode by hot pressing and a loss of area for the interface betweenthe electrocatalyst and the membrane. Although the latter can be partlycompensated for by appropriate impregnation of the electrode with aNAFION solution, the long diffusion path for protons around the gridthrough the recast membrane leads to relatively high local resistances.

FIG. 6(b) shows an M&E configuration 85 with the metal grid 82 in thecenter of the electrode, between the uncatalyzed carbon gas diffusionlayer 58 and the active electrocatalyst layer 56. The advantages of thisconfiguration are good contact between the grid 82 and theelectrocatalyst 56 for efficient current collection and no blockage ofthe interface between the membrane 54 and the active electrocatalyst 56.The disadvantages of this configuration are a more difficult fabricationprocedure and possible interference with gas diffusion inside theelectrode.

FIG. 6(c) shows an M&E configuration 90 with the grid 82 on the backside of the electrode. The advantages of this configuration are goodcontact between the grid 82 and the cell frame 30 (see FIG. 4) forefficient current collection, no blockage of the interface between themembrane and the active electrocatalyst, and simpler fabrication thanthe arrangement with the grid inside the electrode. The disadvantages ofthis arrangement are the creation of vertices on the outer face of theelectrode which can act as anchors for water drops leading to a loss inavailable area for gas diffusion and the difficulty of insuring that thegrid will stay in contact with the electrode for an indefinite period oftime.

Electrochemical Cells with Internal Bipolar Electrically ConductingMember

One aspect of the invention provides an electrically conductive memberdisposed within the electrode, rather than having a conductive clothlayer and a conductive fame disposed along a perimeter face of theelectrode. The electrically conductive member may be a sheet of expandedmetal mesh or wire, preferably expanded metal mesh The expanded metalmesh or other electrically conductive member preferably has a greaterportion of open area than does carbon cloth or paper to increase the gasflow to and from the catalyst areas.

Now referring to FIGS. 30(a-b), the structures of two different M&Eassemblies specifically intended for use in a stack of monopolar fuelcells wherein the internal electrically conductive element 82 of theanode in one cell extends and forms the conducting element in thecathode of an adjacent cell.

The difference between the M&E configurations of FIGS. 30(a-b) and thoseof FIGS. 6(a-b) respectively is the elimination of the conductive carboncloth 84 shown at the back of the electrodes in FIGS. 6(a-b).

The metal grids of FIGS. 30(a-b) provide the support normally providedby the carbon cloth or carbon paper in conventional gas diffusionelectrodes while adding in-plane conductivity to the electrode. Sincethe metal grid is quite close to the membrane in this design, it isimperative that the metal not corrode. Corrosion will not only increasethe contact resistance between the active portion of the electrode andthe current collecting frame, but mobile metal ions coming in contactwith the membrane may replace protons in the membrane. Replacing even asmall fraction of the protons in the membrane with far less mobile metalions will lead to a significant drop in membrane conductivity. Thisfactor imposes a requirement that the grids have low contact resistanceand be corrosion resistant. The best way to impart these properties to apiece of lightweight material is to plate the metal with a layer of amore precious metal, such as gold, platinum, palladium, or ruthenium, toprotect it from corrosion and to improve electrical contacts.

Another advantage to fabricating electrodes with an internal conductiveelement so long as the conductivity of the resultant electrode issufficient, is that the bipolar cell frame described above is no longerneeded for consecutively connecting the cells. Eliminating the cellframe leads to a smaller and lighter stack.

FIG. 7 shows the improvement in monopolar fuel cell performance producedby the inclusion of a metal grid on the back of the electrode. Theconfiguration 90 of FIG. 6(c), with the grid on the back side of theelectrode, gave the best performance of the three designs in FIGS.6(a-c).

FIG. 8 shows a module design 95 that would stack head-to-tail (end 96 ofone module to end 98 of another module) using a combined plug 99 withfour electrical connections 94 and a gas connection 97. Inside themodule, terminal ends of the cells are electrically connected intogroups, each of which will supply power at a potential of 12 Volts (theminimum output that a power system may have for many applications). Themating plug on the item being powered (not shown) can be wired toutilize the groups in parallel for 12 Volts, or in series for 24 Volts.As many modules 95 as desired can be connected together to increase thecurrent available, but the voltage will remain constant at either 12 or24 Volts. It is anticipated that the modules could be stackedside-by-side or end-to-end and operate equally well.

The present invention provides an open fin design that produces a verylight fuel cell system. The power-to-weight ratio for the fins alone (nofittings) has been shown to be as high as 220 W/lb. When the weight ofthe fittings needed to connect the stack to the fuel supply and the nextmodule are included, this drops to a still impressive 180 W/lb. Whilethis is the lightest arrangement, this design is only practical forenclosed applications because the lightweight construction leaves thecell subject to damage.

FIG. 9 shows an even more compact and useful fuel cell configuration100. In this arrangement a plurality of fins 102 are enclosed in acommon housing or shell 104, with a small fan 106 located at one end tomove a continuous stream of air over the cathodes 108. With 4 mm thickfins, a fin-to-fin spacing of 2-3 mm would be required, or about 4 finsper inch. This arrangement produces a projected stack volume of about335 ml for a module sized to produce about 50 Watts of power. It islikely that this volume can be reduced further, with a volume belowabout 300 ml possible.

The fuel is delivered to each anode chamber 110 through a manifold 112,preferably located at the rear of the fins. A fuel source, such as apressurized hydrogen tank and step-down pressure regulator (not shown),is connected to the coupling 114 on the manifold. With hydrogen incommunication with the anodes and air passing over the cathodes 108 andout the back grill 116 of the housing 104, the fuel cell 100 willgenerate electricity for use in any 12 or 24 Volt DC application.

The addition of the fan 106 and the shell 104 increase the weight of thesystem 100 slightly, thereby reducing the power/weight ratio to about150 W/lb for a 50 Watt module. Although the fuel cell 100 represents anincrease in the amount of weight needed for any specified power output,the fuel cell is still quite light and the smaller size makes the cellmuch more portable and easy to carry.

The light weight fuel cell design of the present invention is suitablefor use with gaseous fuels, such as hydrogen, or with liquid fuels, suchas methanol, dimethoxymethane, or trimethoxymethane.

Both of these systems, as shown in FIGS. 8 and 9, are drawn as hydrogenfuel cells, with the gas being supplied through a feed line 94. Thistechnology can be used with liquid fuels as well. In the case of liquidfuels, the water-alcohol mixture consumed by the fuel cell would besupplied using the same common manifold fuel distribution approach asshown for hydrogen. Attitude independence, easily achieved in a gasfueled system, can be achieved in a liquid fueled system through the useof a wicking system to feed the anodes by using capillary action to liftthe fluid from the bottom of a liquid fuel container (not shown), whereever it may reside.

FIG. 10 is an exploded view of a subassembly 120 including a pair of gasis diffusion electrodes 122 fabricated with internal bipolar conductivemetal grids, made in accordance with FIG. 6(b). The electrodes 122 eachcomprise an active cathodic catalyst region 126, an active anodiccatalyst region 128 and a gas barrier 130 disposed therebetween. Aproton exchange membrane 132, preferably having a PTFE frame 134 isdisposed between the cathodic region of one electrode and the anodicregion of another electrode. While each active catalytic region is shownas having the catalyst deposited on one surface of the gas diffusionmatrix, it is also possible to deposit a thin layer electrode on thesurface of the membrane. In the former situation, one surface of the gasdiffusion electrode is catalyzed on one side of the gas barrier and theopposite surface is catalyzed on the other side. In the latter case, thediffusion backing is bonded to the back of the electrode, and the gasdiffusion matrix can be fabricated as a symmetrical unit, i.e., it isnot necessary to deposit catalyst on either side thereof.

In either case, the electrodes are assembled with a PEM membrane betweenthem and a PTFE “window fame” gasket included to eliminate thepossibility of electrodes shorting at the edge due to physical contactof the anode 128 and cathode 126. In this geometry each grid has both ananode and a cathode bonded to it and serves as the bipolar link betweenthem. This eliminates the need for the bipolar cell frames. In thisdesign the conductive elements within the electrodes serve as thebipolar elements connecting each pair of adjacent cells. PTFE frames areincluded to prevent shorting from electrode to electrode at the edge.

The gas barrier strip 130 disposed down the center of the conductivemetal grid can be fabricated in a variety of ways. A polymer strip canbe deposited using a self curing polymer, such as silicones, epoxies,and urethanes; a thermoplastic; or easily melted metals, such as solder.The barrier can also be produced as part of the grid fabrication processwith the gas barrier strip 130 comprising a region of unexpanded metal.It can be readily appreciated by those skilled in the art that othermethods may be used in producing light weight fuel cells, such methodsare considered to be within the scope of the present invention.

Also, a wide variety of materials may be suitable for use as theconductive metal element within the gas diffusion electrode. Thosedescribed here are only examples, the use of materials not describedhere is within the scope of the present invention. The preferredmaterial for the conductive element is expanded metal, a productfabricated by piercing and stretching a sheet of metal or metallic foil.For optimal performance, the metal should be flattened after expansionto restore it to its original thickness.

FIG. 11 shows the basic form of the expanded metal sheet used inconjunction with this invention. Expanded metal foils with the patternshown as well as others are available from a wide variety ofmanufacturers, in a wide range of thickness, from 0.063″ (0.076 mm) to0.1″ (2.5 mm) or more and manufactured from a variety of metals,including, but not limited to, titanium, nickel, copper, stainlesssteel, aluminum, and niobium. They are available with open areas (thepercentage of the sheet area comprised of the holes) ranging from 10% to70%. A typical expanded metal sheet has two primary directions. For bestperformance, the expanded metal sheet should be oriented such that thecurrent flows in the direction parallel to the “long way” of thediamond, since the electrical resistance is lowest in this direction, asshown in Table I, which contains some properties of typical expandedmetals and other materials useful for forming the conductive portion ofthe electrode structures, as taught here. Some of the materials in thistable were gold plated prior electrical measurements so that contactresistance is reduced. TABLE I Properties of Metal Conductive ComponentsAreal Resistivity Resistivity Open Thickness LWD^(a) SWD^(a) density(LWD) (SWD) Fraction Label (cm) (cm) (cm) (g/cm²) (mΩ cm/cm) (mΩ cm/cm)(%) Ni #1 0.028 0.167 0.125 0.01994 6.08 9.62 51 Ni #2 0.028 0.172 0.1250.01977 5.72 10.34 56.5 Ni #3 0.0285 0.325 0.112 0.03046 2.79 10.0 43 Ni#4 0.016 0.186 0.110 0.0493 1.78 4.68 35 Ni D^(b) 0.041 0.185 0.1130.0383 2.62 9.25 41 Ti 77^(b) 0.009 0.138 0.075 0.0093 35 119 52 SS080^(b) 0.014 0.123 0.071 0.0288 33 82 31.4 SS 4/0^(b) 0.009 0.1 0.0790.0185 50 133 34 SS 100^(b) 0.011 0.134 0.1 0.0219 42 123 40 Cu 4/30.0172 0.14 0.0074 0.0545 0.38 1.17 31 Cu 3/0 0.017 0.2 0.09 0.069580.26 0.93 37.8 Ni Foam 0.05 n.a.^(c) n.a.^(c) 0.0853 3.2^(d) n.a.^(c)68^(e) Perf. SS 0.0135 n.a.^(c) n.a.^(c) 0.06 6.7^(d) n.a.^(c) 38^(f)Notes:^(a)LWD and SWD are defined in FIG. 11.^(b)Not gold plated.^(c)Not applicable, this material is not asymmetric.^(d)Resistivity in either direction, material is symmetric.^(e)Calculated.^(f)Manufacturer's value.

Perforated metal sheets are also suitable as the conductive componentwithin the electrode. Compared to expanded metal, perforated metalsheets are generally stronger and more conductive, but generally displaya smaller open fraction (<40%) and are thus less conducive to gasexchange.

Woven metal wire products are suitable as well. Compared to expandedmetal woven products have more open area (up to 80%) for superior gasexchange. However, the weaving process makes woven metal wire productsmore expensive to manufacture than expanded metal, and, becauseelectrical conductivity requires that the current flow through a largenumber of wire-to-wire contacts, resistivity is high. Furthermore,because of the woven nature of the material, any individual sheet canonly be as thin as twice the wire diameter.

Conventional carbon cloth supported gas diffusion electrodes have a gasdiffusion matrix consisting of conductive carbon powder bound together,and to the carbon cloth, by polytetrafluoroethylene (PTFE). Since theexpanded metal has substantially more open area than the carbon cloth, amodification of the gas diffusion matrix formulation would beadvantageous. It has been found that replacing a portion of the carbonpowder with finely cut and divided conductive carbon fibers produces asuperior gas diffusion matrix. The inclusion of conductive carbon fibersin the gas diffusion matrix improves the matrix's ability to span thelarge gaps in the expanded metal, or other open metal, used as theconductive support.

FIGS. 12(a-c) are three views of a fin configured for liquid fuel use.FIG. 12(a) is a top view of a liquid fuel cell system 100 showing thetop view of a fin 101 defining a cathodic surface 103. FIG. 12(b) is across-sectional view of the fin 101 cut vertically near the center ofthe arrangement shown in FIG. 12(a). The anodes 104 in the fin 101define anodic surfaces 105 that are provided with liquid fuel. FIG.12(c) is an enlargement of a portion cut out of the fin 101 as presentedin FIG. 12(b). The wick structure 106 is designed so that all of theanodes 104 are supplied with fuel at all times. Because of the highenergy storage density associated with liquid fuels, the volume insidethe fin allows the storage of sufficient fuel to operate for an extendedperiod of time, from several hours to a day, depending on the load. Thiseliminates the need for any other fuel storage in such a system, unlessan even more extended operating period is required Refueling the liquidfueled system is equally easy, requiring only a container of a pourableliquid.

Fuel cell designs in accordance with the present invention are useful ina wide variety of configurations.

FIGS. 31(a-c) are three views of a fin depicting a compact fuel cellarrangement. In this arrangement, two sets of cells, fabricated using acommon conductive element in the electrodes as the polar element, areattached to opposing faces of a single polymeric frame configured forliquid fuel use.

FIG. 31(a) is a top view of a hydrogen/air fuel cell system 310 showingthe top view of a fin 311 defining a cathodic surface 313

FIG. 31(b) is a cross-sectional view of the fin 311 cut vertically nearthe center of the arrangement shown in FIG. 31(a).

FIG. 31(c) is a cross-sectional view cut horizontally near the center ofFIG. 31(a).

FIG. 25 is a schematic diagram depicting a polymeric frame 230 that canbe used in conjunction with the present invention. The polar framecomprises a first passage 232 for hydrogen feed, a central element 234,and a second passage 236 for hydrogen exit The central element servesboth in preventing contact between the electrodes and channeling thehydrogen in a U-shaped course through a crossover channel 238. It can beeasily appreciated that non-polymeric frames can also be used with thepresent invention. However, when conducting frames are used, they mustbe insulated to prevent the formation of a short circuit when the framecontacts the face of the anodes.

In the arrangement schematically depicted in FIG. 31, a large fractionof the external surface of the fuel cell assembly serves as activecathode area The fins of FIG. 31 can be used as the basic structuralelement in larger arrangements such as the ones depicted in FIGS. 8 and9. These fins can also be operated individually in small power supplies.While the fin of FIG. 31 comprises two sets of 5 cells each for a totalnumber of 10 cells connected in series, there are no limitations on thenumber of cells that can be connected in the manner described here.Arrangements with a larger or a smaller number of cells are consideredwithin the scope of the present invention Also, It is not a necessaryelement of this invention to make symmetrical fins with the same numberof cells on two sides, non-symmetrical arrangements are variationswithin the scope of the present invention. The arrangement described inthis invention presents high flexibility. Adjustments in the number ofcells and their arrangement are provided for by the present invention sothat optimum use is obtained in any particular application.

The significant advantages provided by the compactness in thearrangement presented in this invention may be offset by the difficultyin dissipating the heat generated by operating such compact systems.Another aspect of the invention is to provide means for avoiding theloss in efficiency that may occur when the fuel cell system is operatedfor longer periods of time.

FIG. 26(a) is a cross sectional view of a segment of the fin of FIG.31(b) comprising two membrane and electrode assemblies 244 positionedopposite each other with an internal heat transfer structure 242positioned between the hydrogen feeds 240. The polymer frames 246 arealso shown. FIG. 26(b) is an edge view of the fin of FIG. 12(b) showingthe internal heat structure 242.

FIG. 27 is a drawing depicting a wavy convoluted aluminum sheet 250 asone example of internal heat transfer structures that can be used inconjunction with the present invention. The sheet provides one examplefor the inclusion of a cooling channel and a heat transfer element inthe fin of FIG. 12. In this instance, the sheet is preferably orientedso that the flow direction of the channels in the sheet are parallel tothe longest dimension of the fin. Those skilled in the art of heattransfer can easily appreciate that other orientations of the sheet orthe use of an element made of a different metal or presenting adifferent pattern provide internal heat transfer structures that can beused with the cooling system described here. The presence of an internalheat transfer structure is not a necessary element of the cooling systemdisclosed in this invention. Effective cooling may be obtained even ifthe fin is fabricated with a channel that has no heat transfer element.

While the examples discussed below describe cooling systems that presenta cooling flow parallel to the long direction of the fin, equal coolingefficiency is obtained when the cooling flow is parallel to the shortdirection of the fin. One practical advantage presented by arrangementshaving parallel flow of cooling air and reaction air is the possibilityto use one fan for producing air flow that can be used for bothpurposes. The advantage in using a single fan can be offset whenindependent control of the cooling and reaction flows is desirable. Onthe other hand, fuel systems with cooling and reaction flows that areperpendicular present the advantage of independently controlling theflows but require at least two air moving devices. The choice between anarrangement with parallel cooling and reaction flows and an arrangementwith perpendicular flows depends on the particular application.

Another aspect of the invention is to provide for protection of theexposed cathodes from damage by foreign objects. Such protection can beachieved by adding a protective screen to the arrangement described inFIG. 26. FIG. 28 is a cross sectional view of the fin of FIG. 26(a) withthe back plane of the monopolar cells extended and folded over to form aprotective cover. The fin comprises M&E's 264, hydrogen manifolds 266, acooling channel with a heat transfer element 268, and ventilatedprotective covers 262. In one embodiment of the invention, theprotective cover is formed by extending the metal sheet serving as thebackside of the hydrogen manifold, perforating it, and bending it aroundto form a cover over the faces of the cathodes. By using only a singlesheet of metal as both the backside of the manifold and as theprotective structure, thermal conductivity is maximized. An additionaladvantage of the present design is the contribution of the protectivestructure to heat dissipation during the operation of the fuel cellsystem. The portion of the fin serving as a protective cover comprisesan open area so that efficient air exchange with the atmosphere ismaintained The open area may be obtained by placing perforations in theportion of the fin that serves as a protective cover. Other means formaintaining gas exchange between the cathodes and the atmosphere arewithin the scope of the present invention.

FIG. 29 is a cross sectional view of a fin with protective cooling fins.The cathodes 274 are protected by protecting cooling fins 272 that arebent and form bumpers at the edge of cooling channel 278. In thisarrangement, the bumpers offer protection to the cathodes while servingas the heat transfer element. The arrangement also presents theadvantage of providing a fuel cell system with cathodes that havemaximum access to the atmosphere.

Other arrangements included within the scope of the present inventionare shown in FIGS. 13(a-b). FIGS. 13(a) and 13(b) are side views of twoconfigurations in which the cells are wrapped around the fuel tank 118.FIG. 14 is a cross-sectional view of a monopolar fuel cell system 120.Curved monopolar fuel cell frames 122 and cured membrane and electrodeassemblies 124 are mounted on the outside of a vessel 128 that serves asa fuel reservoir for storing the fuel to be consumed by operating thefuel cell system. When the fuel cylinder is a storage vessel filled withliquid fuel, the operating time for this configuration would be muchlonger than when hydrogen gas is used as the fuel. It is important tonote that in this invention, the conceptual difference in the design ofgas and liquid fueled cells is the addition in liquid fuel cell designsof a wicking element inside the open volume reserved for hydrogendelivery in the gas fueled cell design.

Given the high energy density of liquid fuels, a monopolar fuel cell ofthe design taught here can be fabricated in a wide variety of shapesthat can be integrated into the device that they are intended to power.All of the needed fuel can be stored within the fuel cell structure(i.e., no external tank). For example, a methanol-fueled, 5 Watt flatpanel fuel cell with a 12 cell array, operating at a potential of 0.5V/cell and a current density of 60 mA/cm² occupies an area of about 100cm² and is 7.5 mm thick. The fuel stored inside such a cell is adequatefor over 20 hours of operation.

The examples shown here are all illustrative of fuel cells, but thepresent invention can be useful in other applications as well. Forexample, by filling the wick shown in FIG. 12 with water, the stack canbe operated as an electrolyzer to generate hydrogen at the cathode andoxygen at the anode. It can be readily appreciated that various otherapplications of the concepts described here are within the scope of thepresent invention.

The following examples show the function of this invention and some ofits preferred embodiments.

EXAMPLE 1

This example illustrates a gas diffusion electrode using carbon powderas the conductive material in the metal supported matrix.

A gas diffusion electrode was made by mixing Vulcan XC-72R high surfacearea carbon powder (available from CABOT Technology Division, Pampa,Tex.) with polytetrafluoroethylene “PTFE” suspension (such as T-30,available from DuPont, Wilmington Del.) in a 60:40 ratio (based on thedry weight of the PTFE), water, and a nonionic surfactant (such asTriton X100, available from Fisher Scientific, Fair Lawn, New Jersey).The carbon/PTFE mixture was sonicated to reach complete dispersion andthe resulting paste was spread onto an expanded foil grid. This grid,fabricated from expanded copper foil, had previously been electroplatedwith a protective gold layer, through the use of a standard commercialgold electroplating both (E-56, from Engelhard, Corp., Iselin, N.J.) andserves as the grid to form a gas diffusion electrode.

The bulk of the water was removed from the gas diffusion electrode byfiltration. Polycarbonate filter membranes (5 μm pore size, availablefrom Poretics, Livermore, Calif.) were found to give the most efficientwater removal, with only traces of carbon occasionally passing throughand easy separation of the electrode from the filter. Finer porematerials (1 and 3 μm) blocked all of the carbon, but had much lowerrates for water removal. Other filter materials did not release wellfrom the electrode and retained carbon on the surface of the filter. Theuse of a heat lamp to gently warm the electrode during the filtrationstep was found to improve the water removal. The electrode was dried atroom temperature in a vacuum oven for 4 to 5 hours, then further driedat 60° C. in the same vacuum oven overnight. Finally, the electrode wastreated in an argon atmosphere at 320° C. for 2 hours in order to sinterthe PTFE and decompose the. surfactant. The density and thickness of thefinal gas diffusion electrodes were about 0.57 g/cm³ and 0.7 mm,respectively.

EXAMPLE 2

This example illustrates a gas diffusion electrode using carbon fiberwith carbon powder in the metal supported matrix.

The gas diffusion electrode was made by mixing Vulcan XC-72R highsurface area carbon powder (CABOT) and Thornel™ DKD-X carbon fiber(Amoco Corp., Apharetta, Ga.) with PTFE suspension (T-30, DuPont), in a52:17:31 ratio (based on the dry weight of the PTFE), water, andnonionic surfactant (Triton X100). The carbon/PTFE mixture was sonicatedand fabricated into a metal supported gas diffusion electrode asdescribed in example 1.

EXAMPLE 3

This example illustrates a gas diffusion electrode using carbon fiberwith carbon powder in the metal supported matrix.

The gas diffusion electrode was made by mixing Vulcan XC-72R highsurface area carbon powder (CABOT) and carbon fiber obtained by ballmilling Toray TGP-H paper (Toray Corp., New York, N.Y.) with PTFEsuspension (T-30, DuPont), in a 42:17:41 ratio (based on the dry weightof the PTFE), water, and nonionic surfactant (Triton X100). Thecarbon/PTFE mixture was sonicated and fabricated into a metal supportedgas diffusion electrode as described in example 1.

EXAMPLE 4

This example illustrates a gas diffusion electrode using carbon fiberwith carbon powder in the metal supported matrix.

The gas diffusion electrode was made by mixing Vulcan XC-72R highsurface area carbon powder (CABOT) and Thornel™ DKD-X carbon fiber(Amoco Corp.) with PTFE suspension (T-30, DuPont), in a 33:16:51 ratio(based on the dry weight of the PTFE), water, and nonionic surfactant(Triton X100). The carbon/PTFE mixture was sonicated and fabricated intoa metal supported gas diffusion electrode as described in example 1.

EXAMPLE 5

This example illustrates a gas diffusion electrode using carbon fiberwith carbon powder in the metal supported matrix.

The gas diffusion electrode was made by mixing Vulcan XC-72R highsurface area carbon powder (CABOT) and Thornel™ DKCD-X carbon fiber(Amoco Corp.) with PTFE suspension (T-30, DuPont), in a 40:20:40 ratio(based on the dry weight of the PTFE), water, and nonionic surfactant(Triton X100). The carbon/PTFE mixture was sonicated and fabricated intoa metal supported gas diffusion electrode as described in example 1.

The compositions from the first five examples are summarized in TableII. TABLE II The Compositions Of The Gas Diffusion Electrode in Examples1 through 5 XC-72R carbon Thornel Ground Triton Example powder carbonfibers Toray paper PTFE X100 1 60% 40% 2% of C 2 52% 17% 31% 2% of C 342% 17% 41% 2% of C 4 33% 16% 51% 2% of C 5 40% 20% 40% 2% of C

EXAMPLE 6

This example demonstrates an alternative method for forming the gasdiffusion matrix.

The gas diffusion matrix can also be fabricated by a direct filtrationapproach where the metallic conductor is placed on a polycarbonatefilter and the gas diffusion matrix impregnated into the conductor byfiltration from a more dilute suspension; Vulcan XC-72R high surfacearea carbon powder, conductive carbon fiber (Thornel™ DKD-X), and PTFEsuspension, (T-30), were mixed in a 33:16:51 ratio (based on the dryweight of the PTFE) with water and nonionic surfactant The mixture issonicated to separate and disperse all of the particles into a uniformsuspension. This mixture is then filtered through a cleaned expandednickel conductive element placed on a polycarbonate filter membrane (1μm pore size) to remove water and directly form a gas diffusion layeraround the metallic conductor.

EXAMPLE 7

This example demonstrates a method for forming the active portion of theelectrode.

The thin film catalyst layer for the electrode was made by mixingplatinum black catalyst (such as fuel cell platinum black catalystavailable from Engelhard, Iselin, N.J.) with a 5% solution of 950equivalent weight Nafion® (available from Solution Technology, Inc.,Mendenhall Penn.). Sufficient water was added to produce a viscous, butfluid mixture. The mixture was then sonicated to disperse the platinumblack and give a uniform ink. This ink was brushed directly onto theNafion® membrane and dried.

EXAMPLE 8

This example demonstrates an alternative method for forming the activeportion of the electrode.

The thin film catalyst layer for the electrode was made by mixingplatinum black catalyst (Engelhard's fuel cell grade) with a 5% solutionof 950 equivalent weight Nafion® (available from Solution Technology,Inc., Mendenhall Penn.). Sufficient water was added to produce aviscous, but fluid mixture. The mixture was then sonicated to dispersethe Platinum black and give a uniform ink. This ink was brushed directlyonto a gas diffusion matrix which had been produced as described inExample 7 and dried at under ambient conditions.

EXAMPLE 9

This example demonstrates still another method for forming the activeportion of the electrode.

The catalyst layer for the electrode was made by mixing platinum blackcatalyst (Engelhard's fuel cell grade) with a PTFE suspension (T-30).Sufficient water was added to produce a viscous, but fluid mixture. Themixture was then sonicated to disperse the platinum black and give auniform ink. This ink was brushed directly onto a gas diffusion matrixwhich had been produced as described in Example 7. The electrode wasallowed to dry under ambient conditions until it appeared dry to theeye, then further dried at 60° C. in a vacuum. After vacuum drying theelectrode was heated to 320° C. in a flowing argon atmosphere and heldat that temperature for 10 minutes to sinter the PTFE and decompose thesurfactant.

EXAMPLE 10

This example demonstrates still another method for forming the activeportion of the electrode.

The thin film catalyst layer for the electrode was made by mixingplatinum black catalyst (Engelhard's fuel cell grade) with a 5% solutionof 950 e. w. Nafion® (Solution Technology, Inc.). Sufficient water wasadded to produce a viscous, but fluid mixture. The mixture was thensonicated to disperse the Pt and give a uniform ink. This ink wasbrushed directly onto a Nafion® membrane, and onto a gas diffusionlayer. The two electrocatalyst layers were allowed to dry separatelyunder ambient conditions.

EXAMPLE 11

This example demonstrates the assembly of a membrane and electrodeassembly (M&E) from the parts described above.

Final assembly of the M&E was accomplished by hot pressing thecomponents to bond them together. Two gas diffusion electrodes togetherwith a membrane were placed in a press which had been preheated to 165°C. and compressed at a pressure of 380 lbs/cm². Where Nafion® bondedelectrodes had been used, the M&E was held at that temperature andpressure for approximately five minutes to “cure” the Nafion® solutionto insolubility.

EXAMPLE 12

This example demonstrates the operation of a single cell fuel cellproduced as described above.

The techniques described in examples 2, 10, and 11 were used to produceM&E's for a series of single cells. These cells were operated and theresults obtained are shown by the polarization curves in FIG. 15. All ofthese M&E's were fabricated using Nafion® 105 membrane, with 2 mg Pt/cm²thin film electrodes, and three rather disparate metallic conductors(56.5% open area expanded nickel, 31% open area expanded copper, andfoamed nickel). All three polarization curves were measured with thecell operating under the same conditions, 20 to 25° C. ambienttemperature, with some self-heating of the cell, air supplied by freeconvection, and essentially no pressure is applied to the hydrogen. Asis readily apparent, all three gave essentially the same performance atcurrent densities below 300 mA/cm², with all potential differences lessthan 20 mV. (The region of interest for fuel cells of this type coversthe current densities from 0 to 300 mA/cm². Even at the upper end ofthis region, the stack will generate heat faster than it can be removedby passive means. The ensuing temperature rise will dry out themembrane, reducing performance.) Electrodes were also produced usingperforated stainless steel as the metallic conductor and yielded resultssignificantly inferior to those shown here.

EXAMPLE 13

This example demonstrates the effect of membrane selection on singlecell performance.

The properties of the PEM membrane used to fabricate the M&E's issignificant to their performance. FIG. 16 shows the polarization curvesfor two M&E's operated under the same conditions, (20 to 25° C. ambienttemperature, with some self-heating of the cell, air supplied by freeconvection, and essentially no pressure on the hydrogen), with the onlydifference being the membrane material used to fabricate the M&E. Thecell utilizing the thinner membrane, Nafion® 112, (0.002″, 0:051 mm),clearly out performs the one utilizing the thicker membrane, Nafion®105, (0.005″, 0.127 mm). From the difference in the slopes in the linearregion of the polarization curves it appears that the two have differentinternal resistances, but the two membranes, measured under the sameconditions, have very similar resistances, with the higher specificconductivity of the Nafion® 105 offsetting its increased thickness.

The key to the difference performance is the difference in the thicknessof the two membranes and how it affects water diffusion. In a typicalbipolar fuel cell, both fuel and oxidizer are preferably humidified to adew point near the operating temperature of the stack. This is donetom,insure that the membranes are kept fully saturated with water tomaximize their conductivity. In the monopolar fuel cell, little, if any,water is supplied by the fuel, and electroosmotic drag is constantlymoving the water within the membrane from the anode to the cathode, asit travels with the protons. The dew point of the air is typically 10°C. or more below the ambient temperature, which promotes evaporationfrom a saturated surface. If the cell has heated itself even 5° C. aboveambient, it adds to the difference. Under these conditions water isreadily lost by the cathode of the fuel cell, and the membrane can dryout.

One source of water available to the membrane is the water formed at thecathode. This water can either evaporate away, or be absorbed into themembrane. It is clear that the surface of the membrane in contact withthe cathode will be saturated with water, but the rest of the membranewill only have access to this water if it diffuses away from thecathode, toward the anode. The rate of diffusion increases with areduced length of the diffusion path (i.e. thickness) and a steepconcentration gradient. As a result, the thinner membrane is betterhumidified, and has a lower resistance during operation, for improvedperformance.

EXAMPLE 14

This example demonstrates the assembly of multi-cell modules. Assemblingthe multi-celled modules requires multiple press cycles, and alsorequires a modification of the press geometry, with the addition of apair of blocks to act as a smaller platen than is normally available ina heated press. This permits the cell being bonded to be placed near thecenter of the press for maximum uniformity of both load and heat, whileusing that only one cell is pressed at a time. This arrangement isillustrated in FIG. 15. Although the pins cannot be seen in thiscross-section, which is intended to highlight the M&E, the two smallblocks are kept in alignment by a set of four pins. These pins arefirmly set in the lower block, with the upper block sliding on the pins.The use of these pins keeps the two surfaces precisely aligned throughthe pressing process.

The multi-cell modules produced are like the one illustrated in thecross-section shown in FIG. 18. The key feature of these modules is theuse of a common metal grid for the anode of one cell and the cathode ofthe next cell in the series. This eliminates the need for any additionalcurrent collectors or frames. This effectively reduces the number ofparts needed to assemble a monopolar fuel cell module, as well as thenumber of electrical junctions. Each electrical junction eliminatedreduces the number of potential failure points and the potential forcontact resistances.

In FIG. 18 the gas barrier is shown as being substantially thicker thanthe metal grid. While some assemblies have been produced in this manner,it is by no means a requirement of this invention. A barrier that is thesame thickness as the grid is equally effective.

EXAMPLE 15

This example demonstrates the performance of a two-cell module.

FIG. 19 shows a polarization curve for a two-cell module produced usingM&E's produced as described in example 14, above, using Nafion® 112membranes. The cells in this module had an active area of about 16 cm².This module produced a power output of 85 mW/cm² (for each cell) at acurrent density of 125 mA/cm² (0.678 Volts), the intended operatingpoint. Although the polarization curve extends to a current density ofnearly 400 mA/cm², a current density of approximately 200 mA/cm²represents the highest power that could be sustained. At higher currentdensities the cells gradually heated up until the membranes began todry-out, and the performance declined.

EXAMPLE 16

This example demonstrates the performance of a three-cell module.

A three-cell module was assembled as taught in example 14 and operatedto determine the effect of fuel humidification on performance. FIG. 20shows the effect of fuel humidification on the performance of this threecell module. There is no significant difference at current densitiesbelow 150 mA/cm². Above 150 mA/cm² the use of humidified hydrogenproduced an improvement in the module's performance. This improvementcan be attributed to an improvement in the modules ability to offset thegreater heating rates produced at the higher current densities, with theadditional water entering with the fuel compensating for increasedevaporative losses in two ways: supplying water to keep the membranehumidified, and adding extra water to remove heat by evaporation. Theoutput of the module operating without humidification is shown in FIG.21, with the electrical and thermal contributions separated. It is clearfrom this figure that there is substantial heat production at highercurrent densities, more than can be removed by simple evaporativecooling. This is one of the reasons for designing the system to normallyoperate at current densities of 125 mA/cm², or less.

EXAMPLE 17

This example demonstrates the effect of fuel pressurization on theperformance of a three-cell module.

All of the data in examples 15 and 16 were measured with the fuel atambient pressure. FIG. 22 shows the effect of pressurizing the fuel inthe three cell system. The 0.9 psi back pressure was produced throughthe use of a 24 inch water column, with a slow purge maintained whileoperating this module and the gas exiting against the water pressure.The only region of the polarization curve where pressurization made anydifference was at current densities of 200 mA/cm² and above, where asmall improvement was observed. (At lower current densities, thevoltages matched within ±2 mV.)

EXAMPLE 18

This example demonstrates the effect of air motion on the performance ofa three-cell module.

All of the data shown in FIG. 20 and in FIG. 22 were obtained with airsupplied solely by free convection. In order to evaluate the effect ofadding forced motion and turbulence, a small fan was mounted severalfeet from the side of the module, and this fan used to maintain a streamof air flowing across the face of the module. Since the module isactually recessed into the face of the test fixture the stream did notstrike the cathodes directly, but instead, created a large amount ofturbulence over the cathode. FIG. 23 shows the effect that thisturbulence has on the module's performance.

It is clear that the moving air stream is reducing the output by asignificant amount. There are two possible reasons for this reduction inoutput. The first of these is increased cooling of the module, whichresults in poorer kinetics, and consequently a lower potential. Thesecond is increased drying of the membranes in the module, with thedrying leading to a rise in membrane resistance, and a decline inoutput. Since the drying will also lead to cooling the module, resolvingthese effects is difficult, but it is at least as easy to give this typeof cell design too much air as it is to give it too little air.

EXAMPLE 19

This example illustrates the effect of using a cooling channel insidethe structure of a fin-type monopolar cell structure.

Two sets of five M&Es were produced according to the procedure describedin Example 14. Two frames similar to those shown in FIG. 25 wereattached to sheets of 3 mil (0.003″, 0.76 mm) aluminum using hot meltadhesive (Bemis 3218, Bemis & Associates, Shirley, Mass.). To bond theflame to a sheet of aluminum, a sheet of adhesive was first trimmed tomatch the shape of the polymeric frame. The sheet of adhesive was placedbetween the frame and the metal sheet, and the whole stack placed in aheated press and bonded by pressing for five minutes at 130° C. Themetal polymer assemblies were attached to opposite sides of a sheet ofconvoluted aluminum, similar to the sheet depicted in FIG. 27. Thermallyconductive epoxy (Omegabond® 101, Omega Engineering, Stamford, Conn.)was used in finishing the cooling channel. The fuel cells were theattached to the polymer frames, and connected to a hydrogen supply.

A small fan was connected to the channel and the stack was operatedfirst with the fan in an off position and then with the fan an onposition. FIG. 29 is a graph showing the polarization curves for a stackoperated with and without air flow from a fan. The polarization curveswere obtained while operating the stack in the same conditions apartfrom the air flow from the fan. The stack was held at a current densityof 80 mA/cm² for at least 20 minutes prior to the start of datacollection. The results shown in FIG. 22 clearly indicate that providingan air flow through the cooling channel keeps the stack at a lowertemperature resulting in an improvement of the performance. A gain of520 mV was obtained at a current density of 100 mA/cm². When the fan isnot operated, operation of the cells resulted in a heating of the stack15 to 20° C. above the ambient temperature of the laboratory (25° C.),as determined by measuring the temperature at the surface of one of thecathodes. When the fan is operated, the temperature of the system waskept between 5 and 10° C. above ambient temperature. The difference inthe increase in the temperature resulted in a significantly reduced rateof water loss from the cells, and improved stack potential. It is to benoted here that the fan was connected so that the air only flowedthrough the cooling channel and the cathodes being exposed to the sameair flow in both tests.

While the foregoing is directed to the preferred embodiment of thepresent invention, other and further embodiments of the invention may bedevices without departing from the basic scope thereof, and the scopethereof is determined by the claims which follow.

1. An apparatus comprising an array of cells; each cell comprising: (a)a membrane; (b) an anode comprising a conductive element; and (c) acathode comprising a conductive element; wherein the cathode of one cellshares its conductive element with the anode of an adjacent cell; thesharing of conductive elements establishing an electrical connectionbetween the cells in the array.
 2. The apparatus of claim 1 wherein thecells are electrochemical cells suitable for the direct production ofoxidized and/or reduced products through simultaneous electrochemicaloxidation and reduction of a compound or a mixture of compounds byapplication of an electrical current.
 3. The apparatus of claim 2wherein the electrochemical cells are suitable for the production ofhydrogen and oxygen by hydrolysis of water.
 4. A method for producinghydrogen and oxygen by electrolysis of water in the apparatus of claim3.
 5. The apparatus of claim 1 wherein the cells are fuel cells suitablefor the direct generation of electricity by oxidation of a fuel andreduction of an oxidizer.
 6. The apparatus of claim 5 wherein the fuelis a gas.
 7. The apparatus of claim 6 wherein the fuel is hydrogen gas.8. The apparatus of claim 6 wherein the fuel is a gas mixture comprisinghydrogen.
 9. The apparatus of claim 5 wherein the fuel is a liquid. 10.The apparatus of claim 9 wherein the fuel is an alcohol, an ether or amixture thereof; said mixture being either neat, or in aqueous solution.11. The apparatus of claim 10 wherein the fuel is selected frommethanol, dimethoxymethane, trimethoxymethane, and mixtures thereof. 12.The apparatus of claim 5 wherein the oxidizer is air.
 13. The apparatusof claim 12 wherein the air is provided by unassisted convection. 14.The apparatus of claim 12 further comprising an air mover for assistedconvention.
 15. The apparatus of claim 14 wherein the air is provided ata pressure less than 0.5 psi above ambient pressure.
 16. The apparatusof claim 14 wherein the air mover is a fan.
 17. A low pressure gaselectrode comprising (a) an electrically conducting supportingsubstrate; wherein said substrate comprises at least two opposite faces;a plurality of openings allowing passage from one face to an oppositeface; said openings encompassing between 10% and 90% of the total areaof the substrate; and (b) an electrically conductive porous materialbound to the substrate comprised of a conductive component and a bindingcomponent; and (c) an electrocatalyst layer bound to the substrate. 18.The electrode of claim 17 wherein the binding component is electricallyconductive.
 19. The electrode of claim 17 wherein the binding componentis electrically non conductive.
 20. The electrode of claim 17 whereinthe passage openings represent between 20% and 80% of the total area ofthe substrate.
 21. The electrode of claim 17 wherein the passageopenings represent between 30% and 75% of the total area of thesubstrate.
 22. The electrode of claim 17 wherein the passage openingsrepresent between 24% and 70% of the total area of the substrate. 23.The electrode of claim 17 wherein the electrically conductive porousmaterial is a mixture comprising a powdered component and a fibrouscomponent.
 24. The electrode of claim 23 wherein the powdered componentcomprises carbon.
 25. The electrode of claim 24 wherein the fibrouscomponent comprises carbon.
 26. The electrode of claim 23 wherein thefibrous component comprises carbon.
 27. The electrode of claim 25wherein the ratio of powdered carbon to fibrous carbon is between 1:1and 5:1.
 28. The electrode of claim 27 wherein the ratio is between 2:1and 3:1.
 29. The electrode of claim 23 wherein the powdered component isa metal.
 30. The electrode of claim 19 wherein the binding component isa polymer.
 31. The electrode of claim 30 wherein the polymer is at leastpartially fluorinated.
 32. The electrode of claim 31 wherein the polymeris a fully fluorinated thermoplastic.
 33. The electrode of claim 32wherein the polymer is polytetraauoroethylene.
 34. The electrode ofclaim 17 wherein the ratio of the binding component to the conductivecomponent is between 2:1 and 1:20; preferably between 1:1 and 1:8; andmore preferably between 1:1 and 1:3.
 35. The electrode of claim 17wherein the electrically conductive supporting substrate comprises ametal.
 36. The electrode of claim 35 wherein the substrate is a sheet ofexpanded metal.
 37. The electrode of claim 35 wherein the substrate is apiece of woven wire fabric.
 38. The electrode of claim 37 wherein thesubstrate is a screen.
 39. The electrode of claim 35 wherein theelectrically conductive supporting substrate is a perforated metalsheet.
 40. The electrode of claim 17 wherein an electrocatalyst layer isdeposited on one face of said substrate.
 41. An electrochemical devicehaving an electrochemical cell comprising the electrode of claim
 17. 42.The electrochemical device of claim 41 wherein the electrochemical cellis an electrolytic cell.
 43. The electrochemical device of claim 42wherein the electrolytic cell is an electrolyzer for the generation ofhydrogen and oxygen from water.
 44. The electrochemical device of claim41 wherein the electrochemical cell is a fuel cell for the generation ofelectricity.
 45. The electrochemical device of claim 44 wherein thecells are connected in a series to form an assembly with increasedelectrical potential.
 46. The electrochemical device of claim 45 whereinthe electrochemical cells are arranged in a bipolar configuration. 47.The electrochemical device of claim 45 wherein the electrochemical cellsare arranged in a monopolar configuration.
 48. A dual electrodestructure comprising a first electrode and a second electrode, whereinthe electrodes comprise: (a) an electrically conducting supportingsubstrate; wherein said substrate comprises at least two opposite faces;a plurality of openings allowing passage from one face to an oppositeface; said openings encompassing between 10% and 90% of the total areaof the substrate; (b) an electrically conductive porous material boundto the substrate comprised of a conductive component and a bindingcomponent; (c) an electrocatalyst layer bound to the substrate; andwherein the two electrodes share the same electrically conductingsubstrate.
 49. The dual electrode structure of claim 48 furthercomprising a barrier to gas passage; wherein the barrier is formedbetween the two electrodes by sealing all of the open spaces of theconductive substrate; and wherein the electrodes having theirelectrocatalyst layers bound to opposite sides of the electricallyconducting substrate.
 50. A fuel cell stack comprising a plurality ofdual electrode structures; each dual electrode structure comprising afirst electrode and a second electrode, wherein the electrodes comprise:(a) an electrically conducting supporting substrate; wherein saidsubstrate comprises at least two opposite faces; a plurality of openingsallowing passage from one face to an opposite face; said openingsencompassing between 10% and 90% of the total area of the substrate; (b)an electrically conductive porous material bound to the substratecomprised of a conductive component and a binding component; (c) anelectrocatalyst layer bound to the substrate; and wherein the twoelectrodes share the same electrically conducting substrate; and whereinthe first electrode forms the anode of a first fuel cell and the secondelectrode forms the cathode of an adjacent fuel cell; the fuel cellsbeing connected in series to produce a potential larger than thepotential of each individual fuel cell.
 51. An apparatus comprising (a)a frame having two opposing faces; and (b) a set of first and secondfuel cell stacks of claim 50; wherein the first fuel cell stack isattached to one face of the frame and the second fuel cell stack isattached to the opposite face of the frame; and wherein the set ofstacks and the frame define a volume for internal storage of a reactantto be consumed by operation of the fuel cells; (c) means to supply saidinternally stored reactant; and (d) means to remove undesirable wasteaccumulated by operation of the fuel cell stacks.
 52. An apparatuscomprising: (a) a frame having two opposing faces; (b) a barrier, and(c) a fuel cell stacks of claim 50; wherein the fuel cell stack isattached to one face of the frame and the barrier is attached to theopposite face of the frame; and wherein the stack, the barrier and theframe define a volume for internal storage of a reactant to be consumedby operation of the fuel cells; (d) means to supply said internallystored reactant; and (e) means to remove undesirable waste accumulatedby operation of the fuel cell stacks.
 53. The apparatus of claim 52manufactured from flexible materials, and wherein the barrier has asurface with a curved shape; the apparatus being mounted on the surfaceof the barrier and adopting the shape of the surface.
 54. The apparatusof claim 52 further comprising an enclosed channel for the flow of acoolant to dissipate heat generated by operating the apparatus; andwherein the barrier provides one surface of the channel.
 55. Theapparatus of claim 54 wherein the channel is surrounded by at least oneset of electrochemical cells.
 56. The apparatus of claim 54 furthercomprising a heat transfer element positioned within the channel. 57.The apparatus of claim 54 further comprising means for improving theflow of the coolant through the channel.
 58. The apparatus of claim 52wherein the barrier has a portion that extends beyond the cells; saidportion allowing the dissipation of the heat generated while operatingthe apparatus.
 59. The apparatus of claim 58 wherein the barrier'sportion extending beyond the fuel cells is bent to extendperpendicularly to the plane of the cells and providing a protection toportions of the electrodes that are exposed to the atmosphere.
 60. Theapparatus of claim 59 wherein the barrier is perforated and further bentto form a cover shield to protect portions of the electrodes that areexposed to the atmosphere.
 61. A method of making a porous electrodecomprised of: (a) mixing high surface area carbon powder with a polymerelectrolyte suspension to form a mixture; (b) sonicating said mixture toreach complete dispersion and form a paste; and spreading said paste ona foil grid to form a gas diffusion electrode.